As is known, technology and applications in the THz frequency range have traditionally been restricted to the field of molecular astronomy and chemical spectroscopy. However, recent advances in THz detectors and sources have opened the field to new applications, including homeland security, measurement systems (network analysis, imaging), biological and medical applications (cell characterization, thermal and spectral mapping), material characterization (near-field probing, food industry quality control, pharmaceutical quality control).
Although commercial uses for THz sensors and sources are growing, this growth is somehow limited by the difficulty of providing reliable THz sources, for which traditional semiconductor technology, due to poor electron mobility, has proven not satisfactory.
Use of vacuum electronics instead of semiconductor technology allows to exploit the property of electrons of reaching higher speeds in vacuum than in a semiconductor material, and thus to reach higher operating frequencies (nominally from GHz to THz). The general working principle of vacuum electronic devices is based on the interaction between an RF signal and a generated electron beam; the RF signal imposes a velocity modulation to the electrons of the electron beam permitting an energy transfer from the electron beam to the RF signal.
Conventional old-generation vacuum tubes included thermionic cathodes for generating the electron beam, operating at very high temperature (800° C.-1200° C.), and suffered from many limitations, among which: high electric power requirements, high heating-up time, instability problems and limited miniaturization.
The above limitations have been overcome with the introduction of vacuum devices with a FEA (Field Emission Array) cathode, that has led to significant advantages, in particular for THz frequency amplification, allowing to work at room temperature, and to achieve size reduction down to the micro- and nanometric dimensions. A FEA structure for RF sources was first proposed by Charles Spindt (C. A. Spindt et al., Physical properties of thin-film field emission cathodes with molybdenum cones, Journal of Applied Physics, vol. 47, December 1976, pages 5248-5263), and is usually referred to as the Spindt cathode (or cold cathode, due to the low operating temperature). In particular, Spindt cathode devices consist of micromachined metal field emitter cones or tips formed on a conductive substrate, and in ohmic contact therewith. Each emitter has its own concentric aperture in an accelerating field between an anode and a cathode electrodes; a gate electrode, also known as control grid, is isolated from the anode and cathode electrodes and the emitters by a silicon dioxide layer. With individual tips capable of yielding several tens of microamperes, large arrays can theoretically produce large emission current densities.
Performance of Spindt cathode devices are limited by damaging of the emitting tips due to material wear, and for this reason many efforts have been spent worldwide in searching innovative materials for their production.
In particular, the Spindt structure was much improved by using Carbon Nanotubes (CNTs) as cold cathode emitters (see for example S. Iijima, Helical microtubules of graphitic carbon, Nature, 1991, volume 354, pages 56-58, or W. Heer, A. Chatelain, D. Ugarte, A carbon nanotube field-emission electron source, Science, 1995, volume 270, number 5239, pages 1179-1180). Carbon nanotubes are perfectly graphitized, cylindrical tubes that can be produced with diameters ranging from about 2 to 100 nm, and lengths of several microns using various manufacturing processes. In particular, CNTs may be rated among the best emitters in nature (see for example J. M. Bonard, J.-P. Salvetat, T. Stockli, L. Forrò, A. Chatelain, Field emission from carbon nanotubes: perspectives for applications and clues to the emission mechanism, Applied Physics A, 1999, volume 69, pages 245-254), and therefore are ideal field emitters in a Spindt-type device; many studies have already acknowledged their field emission properties (see for example S. Orlanducci, V. Sessa, M. L. Terranova, M. Rossi, D. Manna, Chinese Physics Letters, 2003, volume 367, pages 109-114).
In this regard, FIG. 1 shows a schematic sectional view of a known Spindt-type cold cathode triode device 1, using CNTs as field emitters. The triode device 1 comprises a cathode structure 2; an anode electrode 3 spaced from the cathode structure 2 by means of lateral spacers 4; and a control gate 5 integrated in the cathode structure 2. The cathode structure 2 with the integrated control gate 5, and the anode electrode 3, are formed separately and then bonded together with the interposition of the lateral spacers 4. The anode electrode 3 is made up of a first conductive substrate functioning as the anode of the triode device, while the cathode structure 2 is a multilayer structure including: a second conductive substrate 7; an insulating layer 8 arranged between the second conductive substrate 7 and the control gate 5; a recess 9 formed to penetrate the control gate 5 and the insulating layer 8 so as to expose a surface of the second conductive substrate 7; and Spindt-type emitting tips 10 (only one of which is shown in FIG. 1, for simplicity of illustration), in particular CNTs, formed in the recess 9 in ohmic contact with the second conductive substrate 7, and functioning as the cathode of the triode device.
During operation, biasing of the control gate 5 allows controlling the flow of electrons generated by the cathode structure 2 towards the anode electrode 3, at the area corresponding to and surrounding the recess 9; the current thus generated is collected by the portion of the anode electrode 3 that is placed over the control gate 5.
In the triode device 1, a triode (or active) area can thus be defined (denoted with 1a in FIG. 1), including the region at, and closely surrounding, the emitting tips 10 and recess 9, in which electrons are generated and collected; and a triode biasing area 1b, as the region outside and external to the triode area 1a, through which biasing signals are conveyed to the same triode area.